In a rolling bearing, since a rolling motion is performed between bearing rings and rolling elements, raceway surfaces of the bearing rings and rolling surfaces of the rolling elements are repeatedly subjected to contact stress. Due to this, materials used to make up the constituent members need to have qualities of being hard, withstanding load, having a long rolling fatigue life, being good with respect to resistance to wear under sliding and the like.
Then in general, steels that are used very often for materials that make up those members are, as bearing steel, a steel that is classified as SUJ2 under the Japan Industrial Standard and are, as case hardening steel, steels corresponding to those classified a SCR420 and SCM420 under the same standard. Since these materials are repeatedly subjected to contact stress as is described above, in order to acquire the required qualities such as rolling fatigue life, quenching and tempering are applied to bearing steel, and quenching and tempering are applied to case hardening steel after carburizing or carbo-nitriding has been applied thereof so as to obtain a hardness of not less than HRC58, not more than HRC64.
On the other hand, since bearings that are used in members auxiliary to an engine such as alternator, electromagnetic clutches, intermediate pulleys, automotive air-conditioner compressor, water pump, planetary gears in a transmission, continuously variable belt and pulley transmission and gas heat pump that is a similar drive system component support rotational shafts that are driven to rotate by being imparted power from an internal combustion engine such as the engine, the bearings are used under high-temperature, high-load, high-vibration and high-speed driving severe conditions.
Sufficient lubricating films are difficult to be formed on rolling surfaces of the bearings that are used under the severe environment, and a large tangential force is applied to the rolling surfaces of the bearings. Due to this, heat generation and surface fatigue are easy to take place as a result of metal contact. In addition, the metal contact facilitates the generation of a newly formed surface (a surface where the texture of steel is exposed). Since this newly formed surface constitutes a catalyst for tribochemical reaction, an addition agent and water that are contained in a lubricating oil are decomposed on the rolling surface of the bearing to thereby facilitate the generation of hydrogen ions. Then, these hydrogen ions so generated are adsorbed to the newly formed surface generated on the rolling surface of the bearing become hydrogen atoms, and hydrogen atoms are then collected to a stress field (in the vicinity of a maximum shearing stress position). Since a location where hydrogen atoms are collected becomes brittle, an early flaking of the rolling bearing is triggered.
In addition, in the event that foreign matters such as metal cut powder, machined dust, burrs, worn powder enter the lubricant in the rolling bearing, the bearing rings and rolling elements are damaged, the lives of the bearings are largely reduced.
Lubricating oils such as automatic transmission fluid (ATF) and continuously variable transmission fluid (CVTF) which have high friction coefficient are used for lubrication of a continuously variable belt and pulley transmission with a view to operating smoothly the torque converter, gear mechanisms, hydraulic mechanisms and wet clutches, with bearings which support an input shaft side pulley and an output shaft side pulley included in the lubrication of the continuously variable belt and pulley transmission.
Since the rolling bearings for continuously variable belt ant pulley transmissions are used under the high-vibration and high-load severe environment and under lubrication by the lubricating oils having high friction coefficient, the tribochemical reaction is easy to take place, and an early flaking is triggered in the rolling bearings.
Here, in order to make difficult the generation of tribochemical reaction, a method is considered in which a lubricating oil having high kinematic viscosity is used instead of adding addition agents such as a pressure reducing agent and a friction control agent in a lubricating oil for use. However, it is not preferable to use such a lubricating oil having high kinematic viscosity as a lubricating oil for continuously variable belt and pulley transmissions from viewpoints of fuel efficiency, power transmission efficiency from the input shaft to the output shaft and shudder life. Due to this, in the case of rolling bearings for continuously variable belt and pulley transmissions, it is effective to prolong the lives of rolling bearings using not a method of changing lubricating oils but a method of strengthening the rolling surface. The following techniques are raised as those based on this idea.
In JP-A-2003-343577, it is proposed that at least one of an inner ring, outer ring and rolling elements is formed of steel containing lots of Cr which can suppress the collection of hydrogen to the stress field and that the total content of carbon and nitrogen, content of retained austenite and hardness of the rolling surface are constituted specifically.
It is described on pages 5 to 8 in a scientific lecture meeting pre-printed document No. 30-02 (the year of 2002) issued by the Automobile Technologies Association Inc. that hydrogen ions generated by virtue of tribochemical reaction are made difficult to be adsorbed on a newly formed surface by forming a Ni film on the rolling surface of a rolling element using the electroplating method.
In addition, the following techniques are raised as techniques for prolonging lives of bearings that are to be used under the aforesaid severe conditions.
Japanese Patent Publication No. 2883460 proposes a bearing steel in which the content of C is 0.65 to 0.90% by mass, the content of Si is 0.15 to 0.50% by mass, the content of Mn is 0.15 to 1.00% by mass, the content of Cr is 2.0 to 5.0% by mass and the content of N is 90 to 200 ppm and which contains at least one of 100 to 500 ppm Al and 50 to 5000 ppm Nb. According to the technique described in this Japanese Patent Publication No. 2883460, an early flaking is made difficult to take place on the rolling surface, and the reduction in toughness after a heat treatment can be suppressed.
Japanese Patent Publication No. 2013772 proposes that the retained austenite volume is made to be not more than 8% by volume and the hardness is made to be not less than HRC 60 by working on a material in which the content of C is 0.95 to 1.10% by mass, the content of Si or Al is 1.0 to 2.0% by mass, the content of Mn is not more than 1.15% by mass, the content of Cr is 0.90 to 1.60% by mass, the balance is Fe and inevitable impurities, and the content of O is not more than 13 ppm to form the material into a predetermined shape and thereafter applying thereto quenching and high-temperature tempering at 230 to 300° C. According to the technique described this Japanese Patent Publication No. 2013772, the dimension stability at high temperatures can be improved, and the reduction in hardness can be prevented.
JP-A-2001-221238 proposes that the hardness of raceway surfaces is made to be not less than HRC 57 and fine carbides of 50 to 500 nm in diameter are dispersed and precipitated thereon by working on a material in which the content of C is 0.4 to 1.2% by mass, the contents of Si and Al are 0.7 to 2.0% by mass, the content of Mn is 0.2 to 2.0% by mass, the content of Ni is 0.1 to 3.0% by mass, the content of Cr is 3.0 to 9.0% by mass, and the equivalent weight of Cr that is calculated by the following equation is 9.0 to 17.0% by mass to form the material into a predetermined shape and thereafter applying thereto quenching and tempering.Cr Equivalent Weight=[Cr]+2[Si]+1.5[Mo] +5[V]+5.5[Al]+1.75[Nb]+1.5[Ti]  (1)
where in the equation (1) above, [Cr], [Si], [Mo], [V], [Al], [Nb] and [Ti] denote the contents (percent by mass) of Cr, Si, Mo, V, Al, Nb and Ti.
According to JP-A-2001-221238, since the minute carbides dispersed and precipitated on the surface of the raceway trap hydrogen, an early flaking is restrained from occurring on the surface of the raceway.
In addition, there are proposed various types of rolling bearings which can keep long lives even in the event that the rolling bearings are used under a lubrication with tramp materials as in a case where foreign matters enter and remain in a lubricant in the interior of the rolling bearings.
For example, Japanese Patent Publication Nos. 22138103 and 2128328 propose techniques in which the stress concentration on to an edge portion of a dent generated by foreign matters is relaxed so as to suppress the generation of cracks to thereby prolong the life of a rolling bearing by regulating the volumes of carbon, retained austenite and carbonitride on surface layers of bearing rings and rolling elements. In addition, Japanese Patent Publication No. 3051944 discloses a bearing component which provides a superior rolling fatigue life by being controlled with respect to steel composition, interior hardness and surface hardness.
On the other hand, needle roller bearings have a small inside diameter to outside diameter ratio. Namely, they are a bearing that has a thin thickness. However, since the needle roller bearings are characterized in that they have a relatively large load capacity for their thickness, the needle roller bearings are widely used at highly loaded locations such as in automotive transmissions and engines.
Hereinafter, for example, planetary gear bearings will be described which rotatably support planetary gears that are used widely in transmissions and the like. In planetary gear bearings, helical gears are generally used for smooth transmission of force from a planetary gear corresponding to an outer ring. Due to this, the running mark of a planetary shaft that corresponds to an inner ring becomes distorted from a force relationship between the planetary gear and the helical gear. Due to this, an uneven force is applied to a needle roller bearing disposed between the planetary gear and the planetary shaft. Consequently, edge load and skew are generated, leading to problems that the life of the bearing is reduced and that seizing is generated.
JP-A-2002-188643 discloses a technique in which at least one of a needle roller bearing and an inner member is made of a steel whose linear expansion coefficient is smaller than that of an outer member or a steel whose average retained austenite volume is not more than 2%, whereby seizing and pry are suppressed which are associated with a tendency to increase the rotation speed to higher speeds.
Furthermore, Japanese Patent Publication No. 2541160 discloses a rolling bearing which is made of an alloy steel in which the volume of retained austenite (γR) and the contents of C, Cr and Mo are regulated with respect to a surface layer of at least one party of bearing rings and rolling elements, and the surface hardness (Hv) of a surface layer that is subjected to carburizing or carbo-nitriding and the volume of retained austenite satisfies a predetermined relationship.
In addition, since the planetary gear bearing is used in the same container as that of gears such as a sun gear, there is caused a problem that hard foreign matters that are produced by virtue of relative contact and slide between gears enter a lubricating oil in the bearing and affects badly the life of the bearing.
Conventionally, with a view to coping with these problems, improvement in materials and normalization of clearance have been implemented. Namely, using a carbo-nitriding processing technique described in NSK Technical Journal No. 656 (the year of 1993), the volume of retained austenite in the surface layer of a bearing steel or carburizing steel is increased and a circumferential clearance and a radial clearance are made appropriate, whereby the stress concentration on to a dent edge is attempted to be relaxed so as to prolong the life of the bearing.
Patent Document No. 1: Japanese Patent Unexamined Publication No. JP-A-2003-343577
Patent Document No. 2: Japanese Patent Examined Publication No. JP-B-2883460
Patent Document No. 3: Japanese Patent Examined Publication No. JP-B-2013772
Patent Document No. 4: Japanese Patent Unexamined Publication No. JP-A-2001-221238
Patent Document No. 5: Japanese Patent Examined Publication No. JP-B-2138103
Patent Document No. 6: Japanese Patent Examined Publication No. JP-B-2128328
Patent Document No. 7: Japanese Patent Examined Publication No. JP-B-205944
Patent Document No. 8: Japanese Patent Unexamined Publication No. JP-A-2002-188643
Patent Document No. 9: Japanese Patent Examined Publication No. JP-B-2541160 Non-Patent Document No. 1: Scientific Lecture Meeting Pre-printed Publication No. 30-02 (the Year of 2002) by Automobiles Technologies Association Inc.
Non-Patent Document No. 2: NSK Technical Journal No. 656 (the Year of 1993)